Hierarchically porous zeolites

ABSTRACT

The present invention concerns Y-type FAU zeolites with hierarchical porosity having an Si/AI atomic ratio strictly greater than 1.4 and less than or equal to 6, having controlled and optimised crystallinity, and having mesoporosity such that the mesoporous outer surface area is between 40 m 2 ·g −1  and 400 m 2 ·g −1 . The present invention also concerns the method for preparing said Y-type FAU zeolites with hierarchical porosity.

The present invention relates to the field of zeolites, moreparticularly hierarchically porous zeolites, and especiallyhierarchically porous zeolites (HPZ) having a faujasite (FAU) structurewith an Si/Al atomic ratio which is strictly greater than 1.4, inparticular Y-type zeolites, referred to hereinafter as HPY.

Synthetic (i.e. non-natural) zeolites are of ever-increasing interest inindustry, as witnessed especially by the numerous recent researchstudies relating to the production of ever more efficient zeolites, withincreasingly simple synthetic processes that are economic and easy toperform.

In recent years, hierarchically porous zeolites (HPZ) have been thesubject of numerous scientific publications and patent applications.Thus, as early as 2005, the process for synthesizing hierarchicallyporous zeolites with good crystallinity (pure phase, observed by XRD)was described in patent application WO 2007/043 731, using a structuringagent of organosilane type.

The product obtained after calcination comprises a zeolite networklinked to a network of mesopores a few nanometres in diameter. Thehydrothermal resistance of this product is much better than that ofmesoporous solids of MCM-41 type, which makes it possible to envisageapplications in which thermal regeneration takes place.

Other methods for preparing hierarchically porous zeolites, i.e. solidscomprising a microporous network of zeolite type linked to a network ofmesopores, have been developed and may be classified in the followingmanner (review by D. P. Serrano, Chem. Soc. Rev., (2013), 42,4004-4035):

-   -   Post-treatment of the zeolite structures which consists in        removing atoms from the zeolite network to create mesopores;        this may be carried out either via acidic treatments which        dealuminize the solid, followed by washing with sodium hydroxide        which removes the aluminium residues formed (J. Perez-Ramirez et        al., Adv. Funct. Mater., (2012), p 1-12) or via treatments that        combine the action of an acid and that of a structuring agent        which promotes the formation of mesopores (cf. WO 2013/106816).    -   “Hard templating method” or “moulding method” which consists in        using a porous network (organic or inorganic) as a mould; this        porous network is placed in contact with a reaction medium that        can form a zeolite network via hydrothermal transformation,        crystallization of the zeolite is performed and the mould is        then removed either by calcination or by dissolution to generate        the mesoporosity (C. J. H. Jacobsen, J. Am. Chem. Soc., (2000),        122, 7116-7117).    -   Zeolitization of amorphous mesoporous solids such as mesoporous        silicas formed according to the sol-gel technique described        by M. Matsukata et al., (Top. Catal., (1999), 9, 77-92).    -   Direct synthesis mentioned at the start using a structuring        agent of the organosilane type, this type of structuring agent        having the particular feature of having, on the one hand,        affinity with the silico-alumina species which form the zeolite        network by virtue of its silane function, and, on the other        hand, of being able to occupy a space with its long-chain        organic function which serves to occupy the space and to create        mesoporosity when it is removed (patent application WO 2007/043        731).

However, even though the solids obtained according to this directsynthetic process do indeed have hierarchical porosity as shown by thenitrogen adsorption isotherms and the transmission microscopy photos(Angew. Chem. Int. Ed., (2012), 51, 1962-1965), it is observed that:

-   -   the micropore volume of these hierarchically porous zeolites is        significantly lower than that of non-mesoporous zeolites,    -   the structuring agent modifies the growth rates of the crystal        faces, which does not allow the size of the crystals to be        correctly controlled,    -   the increase in content of structuring agent directed towards        increasing the mesopore volume leads to a loss of selectivity        for the crystallization of a given zeolite, which results in the        formation of an unwanted mixture of zeolite structures (Y. Meng        et al., Asian Journal of Chemistry, 25 (8), (2013), 4423-4426).

One of the objects of the present invention is to solve at least thesethree major drawbacks noted for direct synthesis using a structuringagent of organosilane type.

Mention may also be made of the following documents, in which the use ofstructuring agents of organosilane type, and of organosilanederivatives, is described, for the purpose of synthesizing varioushierarchically porous zeolite structures including zeolites X and LTA.

Thus, R. Ryoo (Nature Materials, (2006), vol. 5, p. 718 sqq.) describesthe synthesis of LTA having mesoporosity and, later, (K. Cho et al.,Chem. Mater., 21, (2009), 5664-5673) the synthesis of mesoporouszeolites of LTA type and applications thereof in catalysis. Thediffractograms presented in FIG. 2 of the article by K. Cho (see above)show that there is no contaminating crystalline phase. On the otherhand, the decrease in intensity of the peaks, when there is addition ofstructuring agent and a fortiori when its amount increases, proves adegradation of the crystalline framework (low microporosity).

Patent application EP2592049 proposes the synthesis of a zeolite havingvery substantial and well-organized mesoporosity, but with a markeddegradation of the crystalline framework (very low microporosity). Thisprocess uses a specific structuring agent comprising three ammoniumfunctions.

The studies by W. Schwieger (Angew. Chem., Int. Ed., (2012), 51,1962-1965) concern the synthesis of mesoporous zeolite of FAU (X) typeusing a structuring agent. A single example presents the use of TPHAC([3-(trimethoxysilyl)propyl]hexadecyldimethylammonium chloride) asstructuring agent, with a TPHAC/Al₂O₃ mole ratio equal to 0.06.

The article by Y. Meng (Asian Journal of Chemistry, 25 (8), (2013),4423-4426) describes syntheses of mesoporous zeolite LTA using[3-(trimethoxysilyl)propyl]octadecyldimethylammonium chloride (TPOAC),as structuring agent, and presents a study of various syntheticparameters including the amount of structuring agent used, thealkalinity of the reaction medium and the crystallization temperature.

It emerges that an increase in the content of structuring agent whichshould lead to an increase in the mesopore volume also has the effect ofmodifying the growth rates of the zeolite network, thus resulting in theappearance of other zeolite crystalline phases and thus the formation ofmixtures of zeolite structures, which is not desired. Moreover, thediffractograms of FIG. 1 of the said article show a lowering of thecrystallinity.

The abovementioned prior art moreover shows that the micropore volumesare markedly lower than the micropore volumes of equivalentnon-mesoporous zeolites (i.e. zeolites whose mesoporous outer surfacearea as defined below is strictly less than 40 m²·g⁻¹), which is verydetrimental in applications in which a high content of active sites isrequired. What is more, the size of the crystals is subject and cannotbe modified.

Finally, the preparation processes described in the prior art do notappear to be readily industrializable especially on account of the highcosts that they may generate, and on account of the synthesis times,which are proportionately longer the higher the desired mesoporosity.

With regard more particularly to the mesoporous zeolites of Y type, theliterature provides some references regarding the syntheses thereofinvolving post-treatments of zeolites Y.

Thus, for example, the article by U. Lohse et al. (Z. Anorg. Allg.Chem., 476, (1981), 126-135) describes the creation of a system ofmesopores having a size close to 20 nm in a zeolite Y by steam treatmentthen by acid extraction. However, these successive treatments lead to adrastic reduction in the crystallinity of the mesoporous zeolite Y incomparison with the untreated initial zeolite.

In document US 2013/0183229, the post-treatment is carried out byintroducing an amount of Pluronic® (nonionic surfactant) of the sameorder of magnitude (similar amount by weight) as the amount of zeoliteY, then by long liquid-route treatments, followed by several calcinationtreatments.

Documents U.S. Pat. No. 8,486,369 and US 2013/0183231 presentpost-treatments that use a cetyltrimethylammonium (CTA) halide coupledwith an acid, then a steam treatment. However, such post-treatments havethe major drawbacks of reducing both the crystallinity and the microporevolume of the initial zeolites. They also result in the material yieldsbeing drastically reduced. These effects are even more marked when thedesired mesopore volume formed is large.

Another example is illustrated by the studies by D. Verboekend et al.(Advanced Functional Materials, 22(5), (2012), 916-928) which presentzeolites Y having mesopores obtained by a succession of post-treatments.It is indicated (p. 919, left-hand column, 1^(st) §) that thesepost-treatments greatly degrade the microporosity. This degradation ofmicroporosity is not however visible in Table 2, due to the fact thatthe micropore volume is measured by D. Verboekend (ibid.) using thet-plot method that simultaneously measures the volumes of the microporesand of the small mesopores. Measurement of the micropore volume usingthe Dubinin-Raduskevitch equation only takes into account the microporeswith a diameter that is strictly less than 2 nm (cf. “Adsorption bypowders and porous solids”, F. Rouquerol et al., Academic Press, (1999),chap. 8.2.2, pages 224-225).

Application WO 2012/084276 describes a process for preparing amesoporous zeolite Y by various basic post-treatments but to thedetriment of the microporosity. These treatments furthermore result, asclaimed, in an increase of the Si/Al atomic ratio via dealumination.

Although these processes make it possible to prepare hierarchicallyporous zeolites, as shown by the shape of the nitrogen adsorptionisotherms of the solids obtained, it is important to note that theseprocesses use amounts of post-treatment fluids of the same order ofmagnitude as the initial mass of zeolite with numerous long operations.Furthermore, the mass yield of these processes is less than 60%, whichfurther penalizes their production efficiency. These processes aretherefore long, expensive and relatively unproductive.

It therefore appears that these prior art documents that relate to thepreparation of mesoporous Y-type zeolites only propose synthesistechniques that comprise at least one post-treatment step.

The studies by Baoyu Liu et al. (RSC Advances, 3, (2013), 15075-15084)teach the synthesis of hierarchically porous zeolites Y prepared withthe aid of a sacrifical template. However, these zeolites have microporevolumes that are insufficient with regard to the targeted applicationsthat use the said hierarchically porous zeolites Y.

Thus, one aim of the present invention consists in providinghierarchically porous Y-type FAU zeolites combining mesoporosity, highmicropore volume, optimal purity and adjustable crystal sizes and withan Si/Al atomic ration which is strictly greater than 1.4. Another aimof the present invention consists in providing a process that iseconomical, simple and readily industrializable for the preparation ofthe said zeolites.

The inventors have now discovered that it is possible to preparemesoporous Y-type FAU zeolites directly, i.e. without going through thesynthesis of a Y-type FAU zeolite that will then be subjected to one ormore necessary post-treatments according to the prior art in order toobtain a certain mesoporosity. The hierarchically porous Y-type FAUzeolites (HPY) according to the invention have characteristics that arequite advantageous and are thus readily industrializable by a directroute.

Thus, and according to a first aspect, the present invention relates toa hierarchically porous zeolite having the following characteristics:

-   -   Si/Al atomic ratio strictly greater than 1.4 and less than 6,        preferably between 1.5 and 5, limits inclusive, more preferably        between between 1.5 and 3, limits inclusive,    -   micropore volume Vμp, in cm³·g⁻¹, which satisfies the equation        Vμp=Vμp_(R)±15%, where Vμp_(R) represents the micropore volume,        in cm³·g⁻¹, measured under the same conditions, for a zeolite of        the same chemical nature and of the same crystalline structure,        but the mesoporous outer surface area of which is strictly less        than 40 m²·g⁻¹, and    -   mesoporosity such that the mesoporous outer surface area is        between 40 m²·g⁻¹ and 400 m²·g⁻¹, and preferably between 60        m²·g⁻¹ and 200 m²·g⁻¹ and more preferably between 60 m²·g⁻¹ and        150 m²·g⁻¹.

According to a preferred embodiment, the zeolite according to thepresent invention is a zeolite of FAU type, and especially a zeolite FAUof Y type.

The characteristics mentioned above give the zeolite according to thepresent invention improved and entirely surprising and advantageousproperties, when compared with the solely microporous zeolites orzeolites that are both microporous and mesoporous known in the priorart.

According to one preferred aspect of the present invention, thehierarchically porous zeolite has a numerical mean diameter of thecrystals of between 0.1 μm and 20 μm, preferably between 0.1 μm and 10μm, more preferably between 0.5 μm and 10 μm, and more preferentiallybetween 0.5 μm and 5 μm, limits inclusive.

The crystal size of the zeolite according to the present invention isexpressed via the numerical mean diameter of the crystals by observationusing a scanning electron microscope (SEM), as indicated hereinbelow.The present invention also has the advantage of making it possible toadjust and control this crystal size, especially as a function of thesynthetic conditions detailed below.

The hierarchically porous zeolite of the present invention additionallyhas a controlled crystallinity which means that the zeolite comprises apure zeolite phase, and more specifically consists of a single zeolitephase or comprises, preferably consists of, up to 2% by weight, limitinclusive, of only one or of several other zeolite or amorphous phases,known as contaminating phases (crystallinity determined by XRD,technique described below).

Moreover, the hierarchically porous zeolite according to the inventionhas an optimal crystallinity, that is to say a micropore volume Vμpwhich satisfies the equation Vμp=Vμp_(R)±15%, preferably the equationVμp=Vμp_(R)±10%, more preferentially the equation Vμp=Vμp_(R)±5%, andmost preferably the equation Vμp=Vμp_(R)±3, where Vμp_(R) represents themicropore volume measured under the same conditions, for a zeolite ofthe same chemical nature and of the same crystalline structure, which isperfectly crystalline (according to the base ICDD PDF-2, release 2011)but non-mesoporous within the meaning of the invention, i.e. whosemesoporous outer surface area is strictly less than 40 m²·g⁻¹.

In the present description, all the micropore volumes are expressed incm³.V. The expression “zeolite of the same chemical nature and of thesame crystalline structure, which is perfectly crystalline butnon-mesoporous within the meaning of the invention” is understood tomean a zeolite prepared under the same conditions, but for which nospecific treatment has been employed in order to create a mesoporosity,whether this is by a direct route (using a structuring agent asdescribed later on in the present invention) and/or by post-treatment.

By way of example, mention may be made of Zeolite Molecular Sieves by D.W. Breck, John Wiley & Sons, New York, (1973), table 4.26, p. 351, inwhich the micropore volume Vμp_(R) of a non-mesoporous zeolite FAU NaY,with an Si/Al atomic ratio of between 1.5 and 3, which is perfectlycrystalline, is equal to 0.34 cm³ g⁻¹.

The calculations for the micropore volume and for the mesoporous surfacearea are performed by applying the methods known to those skilled in theart from the nitrogen adsorption isotherm by applying, as indicatedlater, the Dubinin-Raduskevitch equation, for the micropore volume andthe Harkins-Jura t-plot equation for the microporous and mesoporoussurface area.

The hierarchically porous zeolites according to the invention make itpossible to reconcile the properties of accessibility to the mesoporouszeolite active sites known in the prior art and those of maximumcrystallinity and microporosity of “standard” zeolites (withoutmesoporosity). Thus, the hierarchically porous zeolites of the presentinvention have unexpected properties and open new perspectives asregards their fields of industrial application.

In addition, the zeolites of the present invention may be subjected toone or more cationic exchanges (for example with alkali metal oralkaline-earth metal salt(s)) as is well known to those skilled in theart and commonly performed on conventional zeolites.

According to another aspect, the present invention relates to theprocess for preparing the hierarchically porous zeolites as have justbeen described. The process of the invention especially has theadvantages of being performed easily, of being readily transposable tothe industrial scale, especially on account of the high syntheticmaterial yields, the robustness of the process and its rapidity.

More precisely, the process for preparing the hierarchically porouszeolite according to the invention comprises at least the followingsteps:

-   a) preparation of a “growth” gel for the preparation of a zeolite    FAU of Y type, by mixing a source of silica with a source of    alumina, at a temperature of between 0° C. and 60° C.,-   b) addition to the growth gel of step a) of at least one nucleating    agent, at a temperature of between 0° C. and 60° C.,-   c) addition to the reaction medium of at least one structuring    agent,-   d) crystallization reaction by increasing the temperature,-   e) filtration and washing of the zeolite crystals obtained, and-   f) drying and calcination.

The growth gel used in step a) is perfectly known to those skilled inthe art and is perfectly defined, for example, in D. W. Breck (ZeoliteMolecular Sieves, John Wiley and Sons, New York, (1973), pp. 277 etseq.).

It should be understood that step c) of addition of structuring agent(s)may be performed at the same time as steps a) and/or b) or alternativelybefore and/or after steps a) and/or b). In all cases, the structuringagent should be present in the reaction medium before thecrystallization step d). However, it is preferred to add the structuringagent after step b). In addition, a lag time (resting time, with orwithout stirring) may be envisaged between steps a), b), c) and d).

The process of the present invention is characterized by the use of thetechnique of seeding with at least one nucleating agent, which is wellknown to those skilled in the art, chosen, for example, from anucleating gel, a crystal, for example a zeolite crystal, a mineralparticle of any nature, for example kaolin, meta-kaolin, or anotherclay, and the like, and also mixtures thereof.

Without wishing to be bound by the theory, it is considered that thenucleating agent promotes the orientation of the synthesis towards thedesired zeolite. In addition, and by virtue of the presence of thenucleating agent, it is possible to use a larger amount of structuringagent than that described in the prior art without disrupting or slowingdown the crystallization of the zeolite network.

According to a preferred aspect, the nucleating agent is a nucleatinggel and, more preferably, the said nucleating gel comprises ahomogeneous mixture of a source of silica (for example sodium silicate),a source of alumina (for example alumina trihydrate), a strong mineralbase, for instance sodium hydroxide, potassium hydroxide or calciumhydroxide, to mention but the main ones and the ones most commonly used,and water.

According to one preferred aspect, the growth gel comprises ahomogeneous mixture of a source of silica (for example sodium silicateor colloidal silica, preferably colloidal silica), a source of alumina(for example alumina trihydrate), a strong mineral base, such as forexample sodium, potassium or calcium hydroxide to mention only the mainand most commonly used ones, and water.

The homogeneity of the mixture may be obtained according to any processthat is well known to those skilled in the art and, for example and in anon-limiting manner, using a paddle stirrer, a mixer, or alternativelyusing a mixer of Archimedean screw type as described in patent EP 0 818418. The stirrers with a high rate of shearing, for example of mixertype, are preferred.

By way of non-limiting example, with an Archimedean screw whose rotationis set at 300 rpm, satisfactory homogeneity is obtained between a fewminutes and a few tens of minutes, generally between 20 and 30 minutes.

The mixture is generally prepared at temperatures of between 0° C. and60° C., and preferably between 10° C. and 40° C., and, for practical andeconomic reasons, the mixture is prepared at room temperature, forexample at 25° C. The homogenization period is then less than two hours.

The process of the present invention is also characterized by theaddition to the growth gel thus obtained of at least one nucleatingagent, and preferably of a nucleating gel according to the conceptdefined in U.S. Pat. No. 3,947,482. The amount of nucleating gel addedmay vary within wide proportions but is generally between 0.1% and 20%,preferably between 0.5% and 15% by weight and more preferably between 1%and 10% by weight, limits inclusive, relative to the weight of thegrowth gel.

When the nucleating agent is a zeolite crystal, it is preferably azeolite crystal of the same nature as the zeolite that it is desired tosynthesize. The size of the crystal may vary within wide proportions,and is, for example, typically between 0.1 μm and 10 μm. According to apreferred embodiment, the zeolite crystal is introduced in the form ofan aqueous suspension. The amount of crystals introduced may also varywithin wide proportions but this amount of crystals is generallytypically between 0.1% and 10% by weight relative to the total weight ofgrowth gel.

As indicated previously, the process of the present invention is aprocess for the direct synthesis of hierarchically porous zeolite, andnot a process in which the hierarchic porosity results from apost-treatment of an already-synthesized zeolite. However, it would notconstitute a departure from the context of the invention to perform asubsequent step of post-treatment of the zeolite as synthesized.

Thus, the process of the present invention comprises a step of additionto the mixture [growth gel/nucleating agent] obtained in step b) of atleast one structuring agent.

The structuring agents that may be used are of any type known to thoseskilled in the art and especially those described in patent applicationWO 2007/043 731. According to a preferred embodiment, the structuringagent is advantageously chosen from organosilanes and morepreferentially from[3-(trimethoxysilyl)propyl]octadecyldimethyl-ammonium chloride,[3-(trimethoxysilyl)propyl]hexadecyldimethylammonium chloride,[3-(trimethoxysilyl)propyl]dodecyldimethylammonium chloride,[3-(trimethoxysilyl)propyl]-octylammonium chloride,N-[3-(trimethoxysilyl)propyl]aniline,3-[2-(2-aminoethylamino)-ethylamino]propyltrimethoxysilane,N-[3-(trimethoxysilyl)propyl]-N′-(4-vinylbenzyl)ethylene-diamine,triethoxy-3-(2-imidazolin-1-yl)propylsilane,1-[3-(trimethoxysilyl)propyl]urea,N-[3-(trimethoxysilyl)propyl]ethylenediamine,[3-(diethylamino)propyl]trimethoxysilane,(3-glycidyloxypropyl)trimethoxysilane, 3-(trimethoxysilyl)propylmethacrylate, [2-(cyclo-hexenyl)ethyl]triethoxysilane,dodecyltriethoxysilane, hexadecyltrimethoxysilane,(3-aminopropyl)trimethoxysilane, (3-mercaptopropyl)trimethoxysilane,(3-chloropropyl)tri-methoxysilane, and also mixtures of two or morethereof in all proportions.

Among the structuring agents listed above,[3-(trimethoxysilyl)propyl]octadecyl-dimethylammonium chloride, orTPOAC, is most particularly preferred.

Use may also be made of structuring agents of higher molar mass, forexample PPDAs (polymer poly-diallyldimethylammonium), PVB (polyvinylbutyral) and other oligomeric compounds known in the field forincreasing the diameter of mesopores.

The amount of structuring agent(s) may vary within wide proportions andin general it is such that the structuring agent(s)/starting Al₂O₃ moleratio is between 0.005 and 0.20, preferably between 0.01 and 0.15, andmore preferably between 0.02 and 0.08, limits inclusive.

The addition of the structuring agent(s) is performed with stirring, forexample as indicated previously in step a), and the mixture is thensubjected to a maturation step, preferentially with stirring, still atthe same temperature, for example at 25° C., for a time ranging from afew minutes to several tens of minutes, typically for one hour, withstirring at 300 rpm.

After this maturation step, the reaction mixture is engaged in thecrystallization step d), with continued stirring, but slower, typicallybetween 20 and 100 rpm, for example at 50 rpm, and while increasing thetemperature up to a value between 60° C. and 100° C., for example 75° C.The time required for the crystallization is generally between a fewhours and several tens of hours, advantageously between 8 hours and 48hours.

After the crystallization step, the zeolite crystals are extracted fromthe reaction medium by filtration, and then washed with one or moresuitable aqueous and/or organic solvent(s), but preferably aqueous, andfinally dried between 50° C. and 150° C., according to the usualtechniques known to those skilled in the art.

The mean size of the crystals may especially be controlled by adjustingthe content of nucleating agent (nucleating gel, or crystals, forexample of zeolite, or the like) relative to the growth gel in step b).

The dried crystals are then subjected to calcination, this step beingnecessary to release both the microporosity (removal of water) and themesoporosity (removal of the structuring agent). The calcinationperformed to remove the structuring agent may be performed according toany calcination method known to those skilled in the art. For example,and in a non-limiting manner, the calcination of the zeolite crystalscomprising the structuring agent may be performed under a stream of anoxidizing and/or inert gas, especially with gases such as oxygen,nitrogen, air, a dry and/or decarbonated air, an oxygen-depleted air,which is optionally dry and/or decarbonated, at one or more temperaturesabove 150° C., typically between 180° C. and 800° C., and preferentiallybetween 200° C. and 650° C., for a few hours, for example between 2 and6 hours. The nature of the gases, the temperature increase ramps and thesuccessive temperature steady stages and the durations thereof will beadapted as a function of the nature of the structuring agent.

It would not constitute a departure from the context of the invention toperform one or more cationic exchanges (for example with alkali metal oralkaline-earth metal salt(s)), before or after the drying and/orcalcination step (step f)), according to the standard cationic exchangetechniques.

As indicated previously, the synthetic process of the invention isperformed easily and is performed in a relatively short time, andespecially in a time reduced by a factor of at least four, relative tothe hierarchically porous zeolites (HPZ) synthetic processes known inthe prior art, which are very long on account of the inhibiting effectof the organosilane structuring agent for the nucleation and the growthof the microporous zeolite network. It has been discovered, entirelysurprisingly, that the inhibiting effect of the structuring agent (forexample TPOAC) is compensated for by virtue of the presence of thenucleating agent.

This simplicity and this speed of synthesis do not, however, harm thequality or properties of the zeolites thus obtained. Specifically, byvirtue of the process of the invention, it is possible to increase theselectivity of the synthesis towards a pure zeolite structure (with lessthan 2% by weight of other contaminating crystalline phase(s)) and to beable to maximize the [micropore volume/mesopore surface area] ratio fora given outer surface area, which is not the case with the knownprocesses of the prior art (cf. for example, the studies by Y. Meng(ibid.) from which it emerges that an increase in the content ofstructuring agent, which should lead to an increase in the mesoporevolume, also had the effect of modifying the rates of growth of thezeolite network, thus resulting in the appearance of other zeolitecrystal phases and thus the formation of mixtures of zeolite structures,which is not desired).

Specifically, with the processes of the prior art, increasing themicropore volume of the zeolite and maintaining a high phase purity areonly obtained by means of very long crystallization times and relativelylow temperatures (<80° C.). However, these processes never achievemicropore volumes comparable to those of the invention.

Thus, when compared with the other HPZ preparation processes, forexample by post-treatment, the process of the invention is moreproductive and less expensive, since it is performed in a single step,over a relatively short time (less than one day) with a small amount ofstructuring agent, and thus globally with a relatively low cost, or atthe very least with a limited excess cost when compared with that of asynthesis of non-mesoporous zeolite, and very much lower than the costinduced by processes of HPZ synthesis via post-treatment.

The use of these hierarchically porous zeolites is particularlyadvantageous in industrial processes such as adsorption, ion exchange,separation, and may also be envisaged in any technical field in whichnon-mesoporous zeolites are usually used.

The present invention is now illustrated by the examples that follow andwhich are presented without any intention to limit the variousembodiments of the invention.

In the examples that follow, the physical properties of the zeolitecrystals are evaluated via the methods known to those skilled in theart, the main ones of which are recalled below.

Loss on Ignition of the Zeolite Crystals

The loss on ignition is determined under an oxidizing atmosphere, bycalcination of the sample in air at a temperature of 950° C.±25° C., asdescribed in standard NF EN 196-2 (April 2006). The measurement standarddeviation is less than 0.1%.

Micropore Volume (Dubinin-Raduskevitch Volume):

The Dubinin-Raduskevitch volume is determined from the measurement ofthe isotherm of adsorption of nitrogen, at its liquefaction temperature.Prior to the adsorption, the zeolite adsorbent is degassed at between300° C. and 450° C. for a time of between 9 hours and 16 hours, undervacuum (P<6.7×10⁻⁴ Pa). The measurement of the adsorption isotherms isthen performed on a machine of ASAP 2020 type from Micromeritics, takingat least 35 measurement points at P/P0 relative ratio pressures ofbetween 0.002 and 1. The micropore volume is determined according toDubinin and Raduskevitch from the isotherm obtained, by applyingstandard ISO 15901-3 (2007). The micropore volume evaluated according tothe Dubinin and Raduskevitch equation is expressed in cm³ of liquidadsorbent per gram of zeolite. The measurement uncertainty is ±0.003 cm³g⁻¹.

Size and Morphology of the Crystals (SEM)

The estimation of the numerical mean diameter of the zeolite crystals isperformed as indicated previously by observation with a scanningelectron microscope (SEM).

In order to estimate the size of the zeolite crystals on the samples, aset of images is taken at a magnification of at least 5000. The diameterof at least 200 crystals is then measured using devoted software, forexample the Smile View software from the publisher LoGraMi. The accuracyis of the order of 3%.

The morphology of the crystals is qualified from SEM photogrHPAs takenat the magnification suited to the size of the crystals (cf. FIG. 1).

Measurement of the Mesopore Outer Surface Area (m²·g⁻¹) Via the t-PlotMethod:

The t-plot calculation method exploits the data of the adsorptionisotherm Q ads=f (P/P0) and makes it possible to calculate the microporesurface area. The mesopore outer surface area may be deduced therefromby determining the difference with the BET surface area which measuresthe total pore surface area in m²·g⁻¹ (BET S=microp. S+mesop. outer S).

To calculate the micropore surface area via the t-plot method, the curveQ ads (cm³·g⁻¹) is plotted as a function of t=thickness of the layerdependent on the partial pressure P/P0 which would be formed on areference non-porous solid (t function of log P/P0: Harkins-Juraequation applied (standard ISO 15901-3:2007)):

[13.99/(0.034−log(P/P0))̂0.5],

in which, in the interval t between 0.35 nm and 0.5 nm, a straight linemay be plotted which defines a y-axis at the origin Q adsorbed whichmakes it possible to calculate the micropore surface area; if the solidis not microporous, the straight line passes through 0.

Observation of the Mesopore Structure by Transmission ElectronMicroscopy (TEM)

The powder is dispersed in ethanol: 1 minute with ultrasonication. Onedrop of the solution is placed on a microscope grate. The sample is leftto dry under the ambient conditions. The observation is performed with atransmission electron microscope (CM 200 from FEI) at a voltage of 120kV.

The magnifications obtained of ×300 000 (cf. FIG. 2) make it possible tovisualize the presence of the mesopores and to estimate their diameters.

Analysis of the Si/Al Atomic Ratio of the Zeolites by X-Ray Fluorescence

An elemental chemical analysis of the hierarchically porous zeolite maybe performed according to various analytical techniques known to thoseskilled in the art. Among these techniques, mention may be made of thetechnique of chemical analysis by X-ray fluorescence as described instandard NF EN ISO 12677: 2011 on a wavelength dispersive spectrometer(WDXRF), for example the Tiger S8 machine from the company Broker.

X-ray fluorescence is a non-destructive spectral technique exploitingthe photoluminescence of atoms in the X-ray range, to establish theelemental composition of a sample. The excitation of the atoms,generally with an X-ray beam or by electron bombardment, generatesspecific radiations after returning to the ground state of the atom. TheX-ray fluorescence spectrum has the advantage of being sparinglydependent on the chemical combination of the element, which offersprecise determination, both quantitatively and qualitatively. Aftercalibration for each oxide, a measurement uncertainty of less than 0.4%by weight is conventionally obtained.

These elemental chemical analyses make it possible to check the Si/Alatomic ratio of the zeolite, the measurement uncertainty of the Si/Alatomic ratio is ±5%.

Qualitative and Quantitative Analyses by X-Ray Diffraction

This analysis makes it possible to identify the crystal phases presentin the analyzed solid since each of the zeolite structures has a uniquediffractogram (or diffraction spectrum) defined by the position of thediffraction peaks and by their relative intensities.

The zeolite crystals are spread out and smoothed on a sample holder bysimple mechanical compression. The acquisition conditions for thediffraction spectrum performed on the D5000 Broker machine are asfollows:

-   -   Cu tube used at 40 kV-30 mA;    -   slit size (divergent, scattering and analysis)=0.6 mm;    -   filter: Ni;    -   rotating sample device: 15 rpm;    -   measurement range: 3°<2θ°<50;    -   increment: 0.02°;    -   counting time per increment: 2 seconds.

The interpretation of the diffraction spectrum (or diffractogram)obtained is performed with the EVA software with identification of thephases using the base ICDD PDF-2, release 2011, which makes it possibleto demonstrate a perfectly crystalline phase.

The quantity of the zeolite X fractions is measured by XRD analysis.This analysis is performed on a Bruker machine, and the quantity ofzeolite fractions is then evaluated by means of the TOPAS software fromthe company Bruker.

EXAMPLE 1 Synthesis of HPY with Addition of Nucleating Gel and GrowthGel with a TPOAC/Al₂O₃ Ratio=0.04

a) Preparation of the Growth Gel in a Reactor Stirred with anArchimedean Screw at 300 rpm.

A growth gel is prepared in a three litre stainless-steel reactorequipped with a heating jacket, a temperature probe and a stirrer, byadding 1446 g of colloidal silica (Ludox AM-30 containing 30% by weightof SiO₂) at 25° C. to a solution of aluminate containing 184 g of sodiumhydroxide (NaOH), 138 g of alumina trihydrate (Al₂O₃.3H₂O, containing65.2% by weight of Al₂O₃) and 800 g of water at 25° C. for 25 minuteswith a stirring speed of 300 rpm.

The stoichiometry of the growth gel is as follows: 2.5 Na₂O/Al₂O₃/8.0SiO₂/117 H₂O. The homogenization of the growth gel is performed withstirring at 300 rpm, for 25 minutes, at 25° C.

b) Addition of the Nucleating Gel

61.2 g of nucleating gel (i.e. 2% by weight) of composition 12Na₂O/Al₂O₃/10 SiO₂/180 H₂O prepared by mixing a sodium silicate with asodium aluminate with stirring for one hour at 40° C., is added to thegrowth gel, at 25° C. with stirring at 300 rpm. After 5 minutes ofhomogenization at 300 rpm, the stirring speed is reduced to 100 rpm andstirring is continued for 30 minutes.

c) Introduction of the Structuring Agent into the Reaction Medium

27.3 g of a solution of TPOAC at 60% in methanol (MeOH) is introducedinto the reaction medium with a stirring speed of 300 rpm (TPOAC/Al₂O₃mole ratio=0.04). A maturation step is performed at 25° C. for one hourat 300 rpm before starting the crystallization.

d) Crystallization

The stirring speed is lowered to 50 rpm and the nominal temperature ofthe reactor jacket is set at 80° C. in order for the temperature of thereaction medium to rise to 75° C. over 80 minutes. After 22 hours at asteady stage of 75° C., the reaction medium is cooled by circulatingcold water in the jacket to stop the crystallization.

e) Filtration/Washing

The solids are recovered on a sinter and then washed with deionizedwater to neutral pH.

f) Drying/Calcination

In order to characterize the product, drying is performed in an oven at90° C. for 8 hours, the loss on ignition of the dried product being 23%by weight.

The calcination of the dried product required to release both themicroporosity (water) and the mesoporosity by removing the structuringagent is performed with the following temperature profile: 30 minutes oftemperature increase to 200° C., then 1 hour at a steady stage of 200°C., then 3 hours of temperature increase to 550° C., and finally 1.5hours of steady stage at 550° C.

A pure mesoporous zeolite Y (identification by X-ray diffractionspectrum), with an Si/Al atomic ratio determined by X-ray fluorescenceequal to 2.6 and with a micropore volume equal to 0.330 cm³·g⁻¹ is thusobtained.

EXAMPLE 2 Synthesis of HPY with Addition of Nucleating Gel and GrowthGel with a TPOAC/Al₂O₃ Ratio=0.02

The process is performed as in Example 1, with a TPOAC/Al₂O₃ mole ratioof 0.02. A pure mesoporous zeolite Y (XRD), with an Si/Al atomic ratiodetermined by X-ray fluorescence equal to 2.6 and with a microporevolume equal to 0.332 cm³·g⁻¹ is thus obtained.

EXAMPLE n° 3 Synthesis of HPY with Addition of Nucleating Gel and GrowthGel with a TPOAC/Al₂O₃ Ratio=0.08

The process is performed as described in Example 1, with a TPOAC/Al₂O₃mole ratio of 0.08. A pure mesoporous zeolite Y (XRD), with an Si/Alatomic ratio determined by X-ray fluorescence equal to 2.6 and with amicropore volume equal to 0.320 cm³·g⁻¹ is thus obtained.

EXAMPLE 4 Synthesis of HPY from a Growth Gel Prepared with a Shear Mixerwith Addition of Nucleating Gel and a TPOAC/Al₂O₃ Ratio=0.06

a) Preparation of the Growth Gel with a Deflocculating Disc (ShearMixer).

The growth gel is prepared in a 3-litre reactor by adding 1136 g ofcolloidal silica (Ludox AM-30 containing 30% by weight of SiO₂) at 25°C. in a solution of aluminate containing 145 g of sodium hydroxide(NaOH), 111 g of alumina trihydrate (Al₂O₃.3H₂O, containing 65.2% byweight of Al₂O₃) and 626 g of water at 25° C. for 3 minutes with astirring speed of 2500 rpm.

The stoichiometry of the growth gel is as follows: 2.5 Na₂O/Al₂O₃/8.0SiO₂/117 H₂O. The homogenization of the growth gel is performed withstirring at 1200 rpm, for 5 minutes, at 25° C.

In order to carry out the crystallisation, the growth gel is transferredinto a 3-litre reactor stirred with an Archimedean screw.

b) Addition of the Nucleating Gel

101 g of nucleating gel (i.e. 5% by weight) of composition 12Na₂O/Al₂O₃/10 SiO₂/180 H₂O having matured for 1 hour at 40° C., is addedto the growth gel, at 25° C. with stirring at 300 rpm. After 5 minutesof homogenization at 300 rpm, the stirring speed is reduced to 100 rpmand stirring is continued for 30 minutes.

c) Introduction of the Structuring Agent into the Reaction Medium

35.2 g of a solution of TPOAC at 60% in methanol (MeOH) is introducedinto the reaction medium with a stirring speed of 300 rpm (TPOAC/Al₂O₃mole ratio=0.06). A maturation step is performed at 25° C. for 1 hour at300 rpm before starting the crystallization.

d) Maturation and Crystallization

A maturation step is performed at 25° C. and at 100 rpm for 10 hours.

The stirring speed is maintained at 100 rpm and a temperature increaseto 95° C. is performed over 2 hours. After a steady stage of 36 hours at95° C., the reaction medium is cooled by circulating cold water in thejacket to stop the crystallization.

e) Filtration/Washing

The solids are recovered on a sinter and then washed with deionizedwater to neutral pH.

f) Drying/Calcination

In order to characterize the product, drying is performed in an oven at90° C. for 8 hours, the loss on ignition of the dried product being 22%by weight.

The calcination of the dried product required to release both themicroporosity (water) and the mesoporosity by removing the structuringagent is performed with the following temperature profile: 30 minutes oftemperature increase to 200° C., then 1 hour at a steady stage of 200°C., then 3 hours of temperature increase to 550° C., and finally 1.5hours of steady stage at 550° C.

327 g of anhydrous zeolite HPY equivalent solid are thus obtained; whichrepresents a yield of 97 mol % relative to the amount of aluminiumengaged. The Si/Al ratio of the HPY determined by X-ray fluorescence isequal to 2.4.

The porosity characteristics (micropore volume, mesoporous outer surfacearea, size of the mesopores) are collated in Table 1.

The morphology of the crystals is presented in FIG. 1 (SEM photo, ×5000magnification) and the mesoporosity is visualized in FIG. 2 (TEM photo,×300 000 magnification). The size of the crystals is between 3 μm and 7μm.

EXAMPLE 5 Synthesis of HPY from a a Growth Gel Prepared with a ShearMixer with Addition of Nucleating Gel and a TPOAC/Al₂O₃ Ratio=0.06

The process is performed as described in Example 4, with addition of 10%by weight of the same nucleating gel relative to the weight of thegrowth gel so as to reduce the size of the crystals.

The zeolite obtained has a crystal size of between 1 μm and 3 μm, i.e.smaller than the size of the zeolite crystals obtained in Example 4.

Comparison of the Characteristics of the Hierarchically Porous Zeolite YPowders Synthesized in Examples 1, 2, 3 and 4

The results of the characterizations of the hierarchically porouszeolites are collated in Table 1 with a comparison with a referencezeolite Y, CBV 100, sold by Zeolyst International, and for which themean size of the crystals is 0.6 μm.

The porosity characteristics (micropore volume, mesopore outer surfacearea, mesopore size) are calculated from the nitrogenadsorption/desorption isotherms at the temperature of liquid nitrogenfor a powder degassed beforehand at 300° C. under vacuum. Themeasurements are taken on an ASAP 2020 machine from Micromeritics.

The micropore volume (cm³·g⁻¹) is calculated according to theDubinin-Raduskevitch theory. The mesopore outer surface area (m²·g⁻¹) iscalculated using the t-plot model. The mesopore size distribution iscalculated via the Density Functional Theory (DFT) method with thecylindrical pore model.

X-ray diffraction makes it possible to identify the crystal phasespresent in the powder from the reference spectra (or diffractograms) ofthe various zeolite structures and to demonstrate the level ofcrystallinity of the solids produced as a function of the peakintensity.

FIG. 3 shows the diffractogram (a) of the reference non-mesoporouszeolite Y (CBV 100) and the diffractogram (b) of the zeolite HPY fromExample 4. This comparison highlights the similarity of the intensitiesof the diffraction peaks between the reference zeolite and the zeoliteof the invention (Example 4). This shows that the crystallinity (andtherefore the micropore volume) is similar in these two zeolites.

The results of the characterizations of the hierarchically porouszeolites (HPY) of Examples 1, 2, 3 and 4 are collated in Table 1 below:

TABLE 1 Synthesis Nitrogen adsorption isotherm at 77 K XRD SynthesisMesop. Mesopore spectrum TPOAC/Al₂O₃ time Vμp outer S size CrystalProducts ratio (h) (cm³ · g⁻¹) (m² · g⁻¹) (nm) phase Zeolite Y CBV100 0— 0.328 20 — Pure FAU HPY Example 1 0.04 26 0.330 100 5 to 10 Pure FAUHPY Example 2 0.02 20 0.332 80 5 to 10 Pure FAU HPY Example 3 0.08 400.320 140 5 to 10 Pure FAU HPY Example 4 0.06 50 0.320 102 5 to 10 PureFAU Key: Zeolite Y CBV 100: reference non-mesoporous zeolite fromZeolyst International. Vμp: micropore volume calculated with theDubinin-Raduskevitch equation. outer S: outer surface area deduced fromthe t-plot extrapolation.

The results presented in Table 1 above show that the morphology of thecrystals varies with the TPOAC content. An explanation is the effect ofthe structuring agent on the growth rates of the various crystal faces.

The synthetic process performed with the use of a seeding gel and anucleating gel makes it possible to vary the micropore volume/mesoporesurface area distribution, while at the same time obtaining a pure FAU(Faujasite) zeolite of Y type, i.e. without observing other crystalforms.

The process described in the present invention is economically viable,simple to perform industrially, with a very substantial saving in timewhen compared with the syntheses described in the prior art. Inaddition, the synthetic process of the invention makes it possible toachieve entirely satisfactory yields, generally greater than 90%relative to the amount of aluminium engaged, which is the element indeficit in the synthesis gel.

1. A hierarchically porous zeolite having at least the followingcharacteristics: Si/Al atomic ratio strictly greater than 1.4 and lessthan 6, micropore volume Vμp, in cm³·g⁻¹, which satisfies the equationVμp=Vμp_(R)±15%, where Vμp_(R) represents the micropore volume, incm³·g⁻¹, measured under the same conditions, for a zeolite of the samechemical nature and of the same crystalline structure, but themesoporous outer surface area of which is strictly less than 40 m²·g⁻¹,and mesoporosity such that the mesoporous outer surface area is between40 m²·g⁻¹ and 400 m²·g⁻¹.
 2. The hierarchically porous zeolite accordingto claim 1, wherein the hierarchically porous zeolite is a Faujasitezeolite.
 3. The hierarchically porous zeolite according to claim 1,having a numerical mean diameter of the crystals of between 0.1 μm and20 μm, limits inclusive.
 4. The hierarchically porous zeolite accordingto claim 1, comprising a pure zeolite phase.
 5. The hierarchicallyporous zeolite according to claim 1, having a micropore volume Vμp whichsatisfies the equation Vμp=Vμp_(R)±10%, where Vμp_(R) represents themicropore volume measured, under the same conditions, for a zeolite ofthe same chemical nature and of the same crystal structure, which isperfectly crystalline but having a mesopore outer surface area which isstrictly less than 40 m²·g⁻¹.
 6. A process for preparing a zeoliteaccording claim 1, comprising at least the following steps: a) preparinga growth gel for synthesizing a FAU zeolite of Y type, by mixing asource of silica with a source of alumina, at a temperature of between0° C. and 60° C., b) adding to the growth gel of step a) at least onenucleating agent, at a temperature of between 0° C. and 60° C., c) tothe reaction medium of at least one structuring agent, d) increasing thetemperature to cause a crystallization reaction and obtain zeolitecrystals, e) filtering and washing the zeolite crystals obtained, and f)drying and calcining the zeolite crystals.
 7. The process according toclaim 6, wherein the nucleating agent is a nucleating gel.
 8. Theprocess according to claim 7, wherein the amount of nucleating gel addedis between 0.1% and 20%, limits inclusive, relative to the weight of thegrowth gel.
 9. The process according to claim 6, wherein the nucleatingagent is a crystal.
 10. The process according to claim 9, wherein theamount of crystal added is between 0.1% and 10% by weight relative tothe total weight of growth gel.
 11. The process according to claim 6,wherein the source of silica is sodium silicate or colloidal silica andthe source of alumina is alumina trihydrate.
 12. The process accordingto claim 6, wherein the structuring agent is an organosilane.
 13. Theprocess according to claim 6, wherein the structuring agent is selectedfrom the group consisting of[3-(trimethoxysilyl)propyl]octadecyldimethylammonium chloride,[3-(tri-methoxysilyl)propyl]hexadecyldimethylammonium chloride,[3-(trimethoxysilyl)propyl]-dodecyldimethylammonium chloride,[3-(trimethoxysilyl)propyl]octylammonium chloride,N-[3-(trimethoxysilyl)propyl]aniline,3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane,N-[3-(trimethoxysilyl)propyl]-N′-(4-vinylbenzyl)ethylenediamine,triethoxy-3-(2-imidazolin-1-yl)propylsilane,1-[3-(trimethoxysilyl)propyl]urea,N-[3-(trimethoxysilyl)propyl]-ethylenediamine,[3-(diethylamino)propyl]trimethoxysilane,(3-glycidyloxypropyl)tri-methoxysilane, 3-(trimethoxysilyl)propylmethacrylate, [2-(cyclohexenyl)ethyl]triethoxysilane,dodecyltriethoxysilane, hexadecyltrimethoxysilane,(3-aminopropyl)trimethoxysilane, (3-mercaptopropyl)trimethoxysilane,(3-chloropropyl)trimethoxysilane, and also mixtures of two or morethereof in all proportions.
 14. The process according to claim 6,wherein the amount of structuring agent(s) is such that the structuringagent(s)/starting/Al₂O₃ mole ratio is between 0.005 and 0.20, limitsinclusive.
 15. The hierarchically porous zeolite according to claim 1,wherein the mesoporous outer surface area is between 60 m²·g⁻¹ and 150m²·g⁻¹.
 16. The hierarchically porous zeolite according to claim 1,wherein the Si/Al atomic ratio is between 1.5 and 3, limits inclusive.17. The hierarchically porous zeolite according to claim 1, wherein thehierarchically porous zeolite is a Faujasite zeolite of Y type.
 18. Thehierarchically porous zeolite according to claim 1, having a numericalmean diameter of the crystals of between 0.5 μm and 5 μm, limitsinclusive.
 19. The hierarchically porous zeolite according to claim 1,consisting of a single zeolite phase.
 20. The hierarchically porouszeolite according to claim 1, having a micropore volume Vμp whichsatisfies the equation Vμp=Vμp_(R)±3%, where Vμp_(R) represents themicropore volume measured, under the same conditions, for a zeolite ofthe same chemical nature and of the same crystal structure, which isperfectly crystalline but having a mesopore outer surface area which isstrictly less than 40 m²·g⁻¹.